The present invention relates to a fluidic, in particular hydraulic, servosystem having an input which can be selectively actuated and an output which is drivingly connected thereto with limited elastic play. A relative movement, which is made possible by the elastic play and is dependent on the magnitude of the forces or moments transmitted between the input and the output, is transmitted between the input and the output as a setting stroke to a servovalve which can additionally also be controlled externally and controls a fluidic servomotor which is drivingly coupled to the output.
In non-track-bound motor vehicles, servosystems configured as power steering systems are used to a great extent to facilitate the steering work of the driver. In such power steering systems, the input is usually a steering wheel or is drivingly connected thereto, whereas the output, assisted by the servomotor, deflects the steered vehicle wheels.
It is known in principle to provide such a power steering system in motor vehicles so that it can be actuated, on the one hand, at will by the driver and, on the other hand, by an external control which makes it possible, for example, to follow a predetermined track or also to carry out obstacle avoidance maneuvers with the vehicle without intervention by the driver.
In a hydraulic power steering system of the type described in DE 3,414,891 C2, the external control intervenes in the hydraulic return flow which normally leads from the servovalve to a hydraulic reservoir. By damming up the return flow, a hydraulic pressure can be produced which, via corresponding control channels, is capable of acting in different ways on mutually opposed end faces of a control slide of the servovalve and thus of adjusting the servovalve. In this way, the servomotor can then be actuated by external control of the servovalve to deflect the steered wheels of the vehicle.
This known system requires servovalves which have a so-called open center, i.e. in a central position are constantly flowed through by the hydraulic medium while feeding the return flow. Additionally, external steering intervention into positions which deviate from the central position of the servovalve is only possible if a return flow also occurs in these valve positions. Typically, however, servovalves through which there is normally a flow block the return flow off in their end positions in order to be able to feed the full power of the hydraulic pressure source to the servomotor if required.
In that known system, there must thus still be a certain return flow even in end positions of the servovalve in order to make external control possible. However, this is the equivalent of the pressure source having to operate with excessive power even in end positions of the servovalve. Moreover, the damming-up of the return flow involves relatively intense heating of the hydraulic medium because additional throttles become active in the dammed state.
DE 2,652,815 C2 combines a power steering system, which can be controlled at will, with an additional hydromotor which can be controlled externally, in which the system is permanently drivingly connected to the wheels which steer the vehicle and is capable of operating independently of the servomotor. Apart from the fact that, in this system, the constructional outlay is undesirably large, it must be taken into account that the additional hydromotor always has to be moved as well in the case of normal intentional steering of the vehicle. Accordingly, this movement as an additional, constantly acting steering resistance, can impede rapid steering manoeuvres.
It is further known from DE 2,818,754 Al to provide the vehicle steering system with an externally controllable servomotor which acts on the steering wheel or the wheel-side part of the steering shaft. In this case, too, the additional steering resistance caused by the servomotor, in particular in the case of rapid steering manoeuvres, is disadvantageous in addition to the large constructional outlay. Moreover, the return of the steering system can be impeded by increased friction in the steering column.
DE 3,303,063 Al and WO 85/02380 relate to hydraulic power steering systems in which the servovalve is a rotary slide assembly whose parts are directly connected to the input side and to the output side of a divided steering shaft. The input-side and output-side parts of the steering shaft are drivingly connected to one another in an elastic manner by a torsion bar. Relative rotations between these parts of the steering shaft are limited by corresponding stops integrated in the servovalve.
An object of the present invention is to provide a servosystem with particularly little constructional outlay, the intention being, in particular, for servovalves with a closed center also to be usable.
This object has been achieved according to the present invention in that parts which are drivingly coupled to the servovalve by an externally controllable setting assembly can be adjusted relative to one another at the input and at the output.
The present invention is based on the general recognition of intervening directly in the drive transmission between the input or output and the servovalve in the case of external control in order to bring about a setting stroke of the servovalve. It is particularly advantageous that servovalves of any desired type can be used in principle, including, in particular, servovalves with a closed center.
According to one presently preferred embodiment of the invention, the externally controllable setting assembly can be effective directly between the input and the output and can make a controlled relative movement possible between the input and the output.
It is further advantageous that the setting assembly can assume a dual function, if required, in that it brings about the relative adjustment between the input and the output, in the case of external control intervention, and thus corresponding actuation of the servomotor. In the case of normal intentional control, the setting assembly effects only a restoring force retroacting on the input and a corresponding restoring moment in order to make a resistance analogous to the respective setting force of the servomotor able to be felt at the input.
According to a further advantageous embodiment of the present invention, the externally controllable setting assembly can be arranged in the drive transmission between the servovalve and the input or output parallel to an elastic transmission element which brings about the elasticity of the play between the input and the output. Thereby, the input and the output are drivingly coupled. In this manner, the setting assembly can operate virtually without power or with little power when adjusting the servovalve because, the elastic tension of the elastic transmission element has no influence on the actuation resistance of the servovalve.
If the servosystem according to the present invention is configured as a power steering system to transmit torques, a rotary slide is preferred as the servovalve between it achieves a particularly compact and simple construction.